Check valves are used in a variety of applications in air compressor systems to allow for the unidirectional passage of upstream pressurized air, that is, pressurized air upstream of the antler of a check valve, above a particular preselected threshold pressure level. The amount of upstream pressure required to initially begin the opening of a check valve against the force of a spring holding it shut is known as the cracking pressure.
Compressor systems are manufactured in a broad range of sizes and capacities that allow for air deliveries that vary from less than 1 Standard Cubic Feet per Minute (“SCFM”) to 100 SCFM and larger. However, individual components of the compressor system, such as attachment fittings, discharge tubes, check valves, and other channeling devices, must be sized and otherwise configured to adequately allow the air delivered by the system's compressor pump to be effectively and continuously removed downstream of the components to prevent a buildup of backpressure which could lead to inefficient system operation or possibly damage to the compressor pump or other system components.
For this reason it is desirable for check valves to be configured to operate with low cracking pressures to prevent a significant portion of the pressure produced by a compressor pump from being lost as back pressure resulting from opening a check valve. Check valves are therefore preferably configured to open and operate with very low cracking pressures. Once partially or fully opened, a check valve must also be capable of allowing air to continuously move downstream to prevent a substantial accumulation of backpressure produced by the compressor pump upstream of the check valve.
Expandable o-ring style check valves are desirable to use since they have an inherent advantage in that they combine a check valve spring and sealing member into one component. However, a number of previous o-ring style check valves, such as those in which air pressure is exerted outwardly against the o-ring in a radial direction, have been limited in that their design inherently requires relatively large cracking pressures for operation. Other types of o-ring style check valves have proven to be unsuited for preventing substantial accumulations of backpressure upstream of the check valve. While some of these valve types have proven to be suitable for allowing for the passage of smaller control flows of air, such as those flow rates that are suitable for performing pneumatically controlled logic operations, they are often unsuitable for allowing the passage of larger process flows of air, such as those used to effect the operation of mechanical devices and fluid-driven processes.
For example, FIG. 1 is a cross sectional view of a bidirectional check valve 30 of the prior art that is configured to allow air pressure from an upstream supply tube 32 passing through a first air space 33 to flow through a single, small first hole 34 to a second air space 36. A single, small second hole 38 allows air pressure to flow from the second air space 36 to the first air space 33. The check valve 30 extends between the first and second air spaces 33 and 36 and a valve seal 39 seals against a divider 37. O-rings 42 and 40 are biased by the spring force of each o-ring 42 and 40 against tapered sections 44 and 46 to normal positions against seats 49 and 48 (as shown) to prevent air flow between the upstream supply tube 32, second air space 36, and first air space 33 through the first hole 34 and second hole 38. Valve pressure chambers 41 and 43 are the spaces that exist between the o-rings 40 and 43, tapered sections 46 and 44, and seats 48 and 49.
When air flows through the first hole 34 or through the second hole 38, the o-rings 42 and 40 move up the tapered sections 44 and 46, respectively, pushed by the air against the spring force of these o-rings to allow air to pass into the first and second air spaces 33 and 36. There is only one first hole 34 and only one second hole 38 to provide paths for the flowing streams of air between the upstream supply tube 32 and second air space 36 and between the second air space 36 and first air space 33, and the sizes of the first hole 34 and second hole 38 are very small compared to the cross sectional size of the upstream supply tube 32. Normally, when a substantial amount of pressure is produced by a compressor pump in an air compressor system and forced through the upstream supply tube 32 toward the check valve 30, the check valve 30 does not allow a sufficient amount of air to move from the supply tube 32 to prevent a substantial accumulation of upstream backpressure, unless the size of the check valve 30 is significantly increased to make the cross sectional sizes of the first and second holes 34 and 38 more proportionate with the cross sectional size of the supply tube 32. Such an increase in size would greatly increase the cost while reducing the practicality of the check valve 30. In the depicted configuration, the relatively small sizes of the first hole 34 and second hole 38 could be sufficient for allowing smaller control flows of air for logic operations, but due to the proportionately larger size of the upstream supply tube 32, it would normally be unsuitable for accommodating larger process flows of air.
Since there is only one first hole 34 and only one second hole 38 to supply air pressure to the valve pressure chambers 43 and 41 for moving each o-ring 42 or 40 along the tapered sections 44 and 46, respectively, if the check valve 30 is incorporated into an air compressor system, in which air is compressed by a reciprocating piston, the normal rapid rise and fall of pressure caused by the piston can cause an uneven or erratic application of force to be applied against each o-ring 42 or 40 in the valve pressure chambers 43 or 41. This may result in an uncontrolled pulsation of the pressurized stream flowing through the check valve 30. The greatest amount of force from the upstream air pressure is applied against each o-ring 42 or 40 at a location nearest the first or second holes 34 or 38, possibly causing each o-ring 42 or 40 to assume an angled position on the tapered section 44 or 46, potentially leading to sticking or uneven wear and stressing of the o-ring 42 or 40.
If the rate of flow and pressure of the air stream that continues to flow from the first hole 34 or second hole 38 into a valve pressure chamber 43 or 41 is too low, a portion of an o-ring 42 or 40 may not remain in a position away from its respective seat 49 or 48, the o-ring 42 or 40 resting completely against the seat 49 or 48 and sealing the valve pressure chamber 43 or 41. Pressure in the valve pressure chamber 43 or 41 will again increase to a level sufficient to force the o-ring 42 or 40 away from the seat 49 or 48 due to the pressure flowing from the first hole 34 or second hole 38. The o-ring 42 or 40 will then in turn again move along the tapered section 44 or 46 away from the seat 49 or 48 and the cycle will be repeated.
FIG. 2 is a cross sectional view of a prior art check valve 50 having a threaded shank member 52 that engages the inside threads 54 of a valve cavity 56. The shank member 52 includes a tapered section 63 on which the o-ring 62 is reciprocally mounted. While FIG. 2 shows the check valve 50 in an open position in which an annular opening 68 allows air to pass between an o-ring 62 and face 64 of the check valve, the o-ring 62 has a spring force that biases the o-ring 62 along the tapered section 63 to seal against the face 64 and prevent the flow of air through the valve 50. The fitting between the shank member 52 and inside threads 54 is sufficiently loose that a leakage clearance 58 exists between the mating threads, permitting air to flow from an upstream position 60 in the check valve 50 past the shank member 52, causing the o-ring 62 to release its seal against the face 64 of the check valve 50 to permit the air to exit the check valve 50. However, since the leakage clearance 58 is small in comparison with the amount of air that would typically be fed by the compressor pump to the valve via the upstream position 60, the leakage clearance 58 cannot alone allow for the passage of a sufficient amount of air from the upstream position 60 to the o-ring 62 to prevent a substantial accumulation of upstream backpressure unless the check valve 30 is increased to an impractically large size. In the depicted configuration, the relatively small size of the leakage clearance 58, while possibly being sufficient for allowing smaller control flows of air for logic operations, would normally be insufficient for accommodating larger process flows of air due to the proportionately larger size of the upstream cross sectional area of the valve cavity 56.
The leakage clearance 58 is also insufficient to supply enough air to cause the o-ring 62 to remain in an open position during operation. A pressure chamber 66 is created between the o-ring 62, shank member 52, and face 64 when the o-ring 62 is closed. Air passing through the leakage clearance 58 increases the air pressure within the pressure chamber 66 to cause the o-ring 62 to move away from its seal against the face 64, creating an annular opening 68 that allows air to exit the check valve 50. However, the annular opening 68 is much larger than the leakage clearance 58 and allows air in the pressure chamber 66 to escape at a rate that is much greater than the rate at which it can be replaced, starving the pressure chamber 66 until it no longer contains sufficient air pressure to force the o-ring 62 up the tapered portion 63 and away from sealing against the face 64. Pressure again begins to increase in the pressure chamber 66 after the o-ring 62 returns to seal against the face 64 and accumulates until it is sufficient to again move the o-ring 62 outward on the tapered section 63. Depending on the application, this repeated process can lead to cycling when the check valve should be open that can cause a pulsating flow of the pressurized stream and premature wear of the o-ring 62 and other components of an air compressor system.
The cracking pressures required for the operation of such check valves can also be substantially greater than a desirable level due to o-ring distortion or limitations on available o-ring surface area. For example, in the check valve 30 of FIG. 1, each o-ring 42 or 40, due to its elastomeric interaction with the tapered sections 44 or 46, is configured to seal against a flat seat 48 or 49 when air does not flow through the first or second holes 34 or 38. Similarly, in the check valve 50 of FIG. 2, the o-ring 62 seals flatly against the face 64 until the pressure chamber 66 accumulates sufficient air pressure to push the o-ring 62 away from the face 64 up the tapered section 63. In both valves 30 and 50, the flattening of elastic o-ring material against the seat 48 or 49 or face 64 reduces the amount of external o-ring surface area that is exposed to the upstream air pressure when each valve 30 or 50 is closed. Since the cracking pressure of an o-ring seal is inversely related to the amount of surface area exposed to airflow that contacts the seal, a reduction in the exposed o-ring surface area significantly increases the cracking pressure of the valve.